// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using Microsoft.Extensions.Primitives;
using Microsoft.Net.Http.Headers;

namespace Microsoft.AspNetCore.Http
{
    /// <summary>
    /// Represents a wrapper for RequestHeaders and ResponseHeaders.
    /// </summary>
    public class HeaderDictionary : IHeaderDictionary
    {
        private static readonly string[] EmptyKeys = Array.Empty<string>();
        private static readonly StringValues[] EmptyValues = Array.Empty<StringValues>();
        private static readonly Enumerator EmptyEnumerator = new Enumerator();
        // Pre-box
        private static readonly IEnumerator<KeyValuePair<string, StringValues>> EmptyIEnumeratorType = EmptyEnumerator;
        private static readonly IEnumerator EmptyIEnumerator = EmptyEnumerator;

        /// <summary>
        /// Initializes a new instance of <see cref="HeaderDictionary"/>.
        /// </summary>
        public HeaderDictionary()
        {
        }

        /// <summary>
        /// Initializes a new instance of <see cref="HeaderDictionary"/>.
        /// </summary>
        /// <param name="store">The value to use as the backing store.</param>
        public HeaderDictionary(Dictionary<string, StringValues>? store)
        {
            Store = store;
        }

        /// <summary>
        /// Initializes a new instance of <see cref="HeaderDictionary"/>.
        /// </summary>
        /// <param name="capacity">The initial number of headers that this instance can contain.</param>
        public HeaderDictionary(int capacity)
        {
            EnsureStore(capacity);
        }

        private Dictionary<string, StringValues>? Store { get; set; }

        [MemberNotNull(nameof(Store))]
        private void EnsureStore(int capacity)
        {
            if (Store == null)
            {
                Store = new Dictionary<string, StringValues>(capacity, StringComparer.OrdinalIgnoreCase);
            }
        }

        /// <summary>
        /// Get or sets the associated value from the collection as a single string.
        /// </summary>
        /// <param name="key">The header name.</param>
        /// <returns>the associated value from the collection as a StringValues or StringValues.Empty if the key is not present.</returns>
        public StringValues this[string key]
        {
            get
            {
                if (Store == null)
                {
                    return StringValues.Empty;
                }

                if (TryGetValue(key, out StringValues value))
                {
                    return value;
                }
                return StringValues.Empty;
            }
            set
            {
                if (key == null)
                {
                    throw new ArgumentNullException(nameof(key));
                }
                ThrowIfReadOnly();

                if (value.Count == 0)
                {
                    Store?.Remove(key);
                }
                else
                {
                    EnsureStore(1);
                    Store[key] = value;
                }
            }
        }

        StringValues IDictionary<string, StringValues>.this[string key]
        {
            get { return this[key]; }
            set
            {
                ThrowIfReadOnly();
                this[key] = value;
            }
        }

        /// <inheritdoc />
        public long? ContentLength
        {
            get
            {
                long value;
                var rawValue = this[HeaderNames.ContentLength];
                if (rawValue.Count == 1 &&
                    !string.IsNullOrEmpty(rawValue[0]) &&
                    HeaderUtilities.TryParseNonNegativeInt64(new StringSegment(rawValue[0]).Trim(), out value))
                {
                    return value;
                }

                return null;
            }
            set
            {
                ThrowIfReadOnly();
                if (value.HasValue)
                {
                    this[HeaderNames.ContentLength] = HeaderUtilities.FormatNonNegativeInt64(value.GetValueOrDefault());
                }
                else
                {
                    this.Remove(HeaderNames.ContentLength);
                }
            }
        }

        /// <summary>
        /// Gets the number of elements contained in the <see cref="HeaderDictionary" />;.
        /// </summary>
        /// <returns>The number of elements contained in the <see cref="HeaderDictionary" />.</returns>
        public int Count => Store?.Count ?? 0;

        /// <summary>
        /// Gets a value that indicates whether the <see cref="HeaderDictionary" /> is in read-only mode.
        /// </summary>
        /// <returns>true if the <see cref="HeaderDictionary" /> is in read-only mode; otherwise, false.</returns>
        public bool IsReadOnly { get; set; }

        /// <summary>
        /// Gets the collection of HTTP header names in this instance.
        /// </summary>
        public ICollection<string> Keys
        {
            get
            {
                if (Store == null)
                {
                    return EmptyKeys;
                }
                return Store.Keys;
            }
        }

        /// <summary>
        /// Gets the collection of HTTP header values in this instance.
        /// </summary>
        public ICollection<StringValues> Values
        {
            get
            {
                if (Store == null)
                {
                    return EmptyValues;
                }
                return Store.Values;
            }
        }

        /// <summary>
        /// Adds a new header item to the collection.
        /// </summary>
        /// <param name="item">The item to add.</param>
        public void Add(KeyValuePair<string, StringValues> item)
        {
            if (item.Key == null)
            {
                throw new ArgumentNullException("The key is null");
            }
            ThrowIfReadOnly();
            EnsureStore(1);
            Store.Add(item.Key, item.Value);
        }

        /// <summary>
        /// Adds the given header and values to the collection.
        /// </summary>
        /// <param name="key">The header name.</param>
        /// <param name="value">The header values.</param>
        public void Add(string key, StringValues value)
        {
            if (key == null)
            {
                throw new ArgumentNullException(nameof(key));
            }
            ThrowIfReadOnly();
            EnsureStore(1);
            Store.Add(key, value);
        }

        /// <summary>
        /// Clears the entire list of objects.
        /// </summary>
        public void Clear()
        {
            ThrowIfReadOnly();
            Store?.Clear();
        }

        /// <summary>
        /// Returns a value indicating whether the specified object occurs within this collection.
        /// </summary>
        /// <param name="item">The item.</param>
        /// <returns>true if the specified object occurs within this collection; otherwise, false.</returns>
        public bool Contains(KeyValuePair<string, StringValues> item)
        {
            if (Store == null ||
                !Store.TryGetValue(item.Key, out StringValues value) ||
                !StringValues.Equals(value, item.Value))
            {
                return false;
            }
            return true;
        }

        /// <summary>
        /// Determines whether the <see cref="HeaderDictionary" /> contains a specific key.
        /// </summary>
        /// <param name="key">The key.</param>
        /// <returns>true if the <see cref="HeaderDictionary" /> contains a specific key; otherwise, false.</returns>
        public bool ContainsKey(string key)
        {
            if (Store == null)
            {
                return false;
            }
            return Store.ContainsKey(key);
        }

        /// <summary>
        /// Copies the <see cref="HeaderDictionary" /> elements to a one-dimensional Array instance at the specified index.
        /// </summary>
        /// <param name="array">The one-dimensional Array that is the destination of the specified objects copied from the <see cref="HeaderDictionary" />.</param>
        /// <param name="arrayIndex">The zero-based index in <paramref name="array" /> at which copying begins.</param>
        public void CopyTo(KeyValuePair<string, StringValues>[] array, int arrayIndex)
        {
            if (Store == null)
            {
                return;
            }

            foreach (var item in Store)
            {
                array[arrayIndex] = item;
                arrayIndex++;
            }
        }

        /// <summary>
        /// Removes the given item from the the collection.
        /// </summary>
        /// <param name="item">The item.</param>
        /// <returns>true if the specified object was removed from the collection; otherwise, false.</returns>
        public bool Remove(KeyValuePair<string, StringValues> item)
        {
            ThrowIfReadOnly();
            if (Store == null)
            {
                return false;
            }

            if (Store.TryGetValue(item.Key, out var value) && StringValues.Equals(item.Value, value))
            {
                return Store.Remove(item.Key);
            }
            return false;
        }

        /// <summary>
        /// Removes the given header from the collection.
        /// </summary>
        /// <param name="key">The header name.</param>
        /// <returns>true if the specified object was removed from the collection; otherwise, false.</returns>
        public bool Remove(string key)
        {
            ThrowIfReadOnly();
            if (Store == null)
            {
                return false;
            }
            return Store.Remove(key);
        }

        /// <summary>
        /// Retrieves a value from the dictionary.
        /// </summary>
        /// <param name="key">The header name.</param>
        /// <param name="value">The value.</param>
        /// <returns>true if the <see cref="HeaderDictionary" /> contains the key; otherwise, false.</returns>
        public bool TryGetValue(string key, out StringValues value)
        {
            if (Store == null)
            {
                value = default(StringValues);
                return false;
            }
            return Store.TryGetValue(key, out value);
        }

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>An <see cref="Enumerator" /> object that can be used to iterate through the collection.</returns>
        public Enumerator GetEnumerator()
        {
            if (Store == null || Store.Count == 0)
            {
                // Non-boxed Enumerator
                return EmptyEnumerator;
            }
            return new Enumerator(Store.GetEnumerator());
        }

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>An <see cref="IEnumerator" /> object that can be used to iterate through the collection.</returns>
        IEnumerator<KeyValuePair<string, StringValues>> IEnumerable<KeyValuePair<string, StringValues>>.GetEnumerator()
        {
            if (Store == null || Store.Count == 0)
            {
                // Non-boxed Enumerator
                return EmptyIEnumeratorType;
            }
            return Store.GetEnumerator();
        }

        /// <summary>
        /// Returns an enumerator that iterates through a collection.
        /// </summary>
        /// <returns>An <see cref="IEnumerator" /> object that can be used to iterate through the collection.</returns>
        IEnumerator IEnumerable.GetEnumerator()
        {
            if (Store == null || Store.Count == 0)
            {
                // Non-boxed Enumerator
                return EmptyIEnumerator;
            }
            return Store.GetEnumerator();
        }

        private void ThrowIfReadOnly()
        {
            if (IsReadOnly)
            {
                throw new InvalidOperationException("The response headers cannot be modified because the response has already started.");
            }
        }

        /// <summary>
        /// Enumerates a <see cref="HeaderDictionary"/>.
        /// </summary>
        public struct Enumerator : IEnumerator<KeyValuePair<string, StringValues>>
        {
            // Do NOT make this readonly, or MoveNext will not work
            private Dictionary<string, StringValues>.Enumerator _dictionaryEnumerator;
            private bool _notEmpty;

            internal Enumerator(Dictionary<string, StringValues>.Enumerator dictionaryEnumerator)
            {
                _dictionaryEnumerator = dictionaryEnumerator;
                _notEmpty = true;
            }

            /// <summary>
            /// Advances the enumerator to the next element of the <see cref="HeaderDictionary"/>.
            /// </summary>
            /// <returns><see langword="true"/> if the enumerator was successfully advanced to the next element;
            /// <see langword="false"/> if the enumerator has passed the end of the collection.</returns>
            public bool MoveNext()
            {
                if (_notEmpty)
                {
                    return _dictionaryEnumerator.MoveNext();
                }
                return false;
            }

            /// <summary>
            /// Gets the element at the current position of the enumerator.
            /// </summary>
            public KeyValuePair<string, StringValues> Current
            {
                get
                {
                    if (_notEmpty)
                    {
                        return _dictionaryEnumerator.Current;
                    }
                    return default(KeyValuePair<string, StringValues>);
                }
            }

            /// <inheritdoc />
            public void Dispose()
            {
            }

            object IEnumerator.Current
            {
                get
                {
                    return Current;
                }
            }

            void IEnumerator.Reset()
            {
                if (_notEmpty)
                {
                    ((IEnumerator)_dictionaryEnumerator).Reset();
                }
            }
        }
    }
}
